Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a probe acting as a waveguide. The probe is generally arranged to extend vertically from the top towards the bottom of the tank. The probe may also be arranged in a measurement tube, a so-called chamber, that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the frequency difference between a transmitted frequency-modulated signal and its reflection at the surface. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
In several applications, the filling level determined using a radar level gauge system is used to determine an amount of funds to be transferred from one party to another. An example of such an application is in the so-called upstream petroleum industry, where GWR systems are often used to determine the filling level in production tanks. This filling level may in turn be used to determine payments to subsurface rights owners.
In this and other applications, it is thus important that the accuracy of the filling level determination is trusted by all parties. Verification of the measurement accuracy of the GWR systems is therefore regularly carried out.
Currently, such verification is performed by pumping the product up and down in the tank and verifying measurement results obtained using the GWR system with manual measurements, such as by dipping.
This type of verification measurement is, however time-consuming and cumbersome. Verification of one level gauge system using this method may require two operators and at least one tank truck during one day.